Lubricating oil compositions for automotive engines have evolved over the years to include a variety of additives to enhance performance. In recent years environmental concerns have lead to ever stricter limits on chemical emissions whilst consumer pressure leads to ever more demanding performance requirements.
There are many types of lubricating oil composition additives used to enhance engine performance. Whilst a particular additive may exhibit benefits in one aspect of engine performance that same additive may also exhibit detrimental effects in another aspect.
One of the most effective antioxidant and antiwear agents, from both a performance and cost-effectiveness standpoint, used conventionally in lubricating oil compositions for internal combustion engines comprises dihydrocarbyl dithiophosphate metal salts. The metal may be an alkali or alkaline earth metal, or zinc, aluminum, lead, tin, molybdenum, manganese, nickel or copper. Of these, zinc salts of dihydrocarbyl dithiophosphate (ZDDP) are most commonly used. While such compounds are particularly effective antioxidants and antiwear agents such compounds introduce phosphonis, sulfur and ash into the engine that can contribute to deleterious exhaust emissions. Thus levels of phosphorous, sulfur and ash in a lubricating oil composition are now strictly controlled in order to reduce environmental impact. In particular, dihydrocarbyl dithiophosphate metal salts contribute significantly towards the phosphorous content of a lubricating oil composition.
In order to reduce the phosphorous content of a lubricating oil composition it is usual to limit the amount of dihydrocarbyl dithiophosphate metal salts in the lubricant. However, it is proving difficult to reduce the amount of dihydrocarbyl dithiophosphate metal salts in lubricating oil compositions without causing an unacceptable reduction in engine performance.
In the past sulfur containing compounds were considered for their antioxidancy properties, but were not favoured over dihydrocarbyl dithiophosphate metal salts due to the sulfur content and their association with copper corrosion and poor nitrile elastomer seals compatibility.
U.S. Pat. No. 5,840,672 discloses an antioxidant system for a fully formulated lubricant comprising a sulfur containing compound which is stated to exhibit excellent nitrile elastomer seals compatibility. The antioxidant composition comprises a combination of (A) a secondary diarylamine, (B) at least one sulfurised olefin and or sulfurised hindered phenol and (C) at least one molybdenum compound. Typically, the molybdenum compound is present in an amount sufficient to provide the lubricating oil composition with from 60 to 1000 ppm of molybdenum. U.S. Pat. No. 5,840,672 postulates that the sulfur containing compound can be used in the composition without detrimental effect on the nitrile elastomer seals. It is clear from the disclosure of U.S. Pat. No. 5,840,672 that the combination of all three elements of this composition is essential in order to achieve the antioxidancy performance without the detrimental nitrile seals performance. It would seem that the molybdenum compound is acting as a sulfur scavenger in the composition of U.S. Pat. No. 5,840,672 and thus controlling the amount of active sulfur present in the lubricant and thereby the nitrile seals performance.
It is an object of preferred embodiments of the present invention to provide an alternative means of attaining antioxidancy performance without detriment to nitrile seals performance and without causing metal corrosion.